I make the distinction that the Universe as we define it is everything that's physically observable and of substance as we know it. So the Universe can’t be infinite since there must be something beyond the Universe by definition. Otherwise there’s no real need to define the term “Universe” if it’s in fact everything. It’s like saying you can have an infinite number of atoms. Not so – impossible since infinity is not a number but a mathematical term to define an imaginary point you can never reach in the number space.
Another way of viewing it is the Universe is like the surface of a sphere. If you travel in a straight line you eventually come back to where you started. So, there must be something beyond the Universe, just like there’s something beyond the surface of the sphere if you jump into the next dimension and go up or down into the sphere.
What’s beyond the Universe? Something other than what we define as the Universe of course.
Of course this is all pure speculation, as no one really knows. If we did we wouldn’t be having this discussion.
